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Wow is all I can say about cesium. This substance is as close to liquid gold as you'll ever see, and it really does literally melt in your hand in a matter of seconds. (Provided it's in a sealed glass ampule: If it were in your hand unprotected it would violently burn away your skin and flesh down to the bone, which it would probably chew up as well.) The alkali metals become progressively more reactive as you go down the periodic table, and cesium is the last non-radioactive one. I can only imagine what would happen if you were to throw some in water. (See sodium for more about what happens with alkali metals and water.)
Cesium is used as the definitive worldwide time standard. The "atomic watches" you see advertised contain small radio receivers which pick up a signal broadcast from Boulder, Colorado (if you're in the US), and use it to set their time. That signal is based on the time from a cesium fountain clock operated by NIST (the National Institute for Standards and Technology).
The clever thing about using cesium is that time is actually defined in terms of cesium. The international unit of the second is defined as "the duration of 9,192,631,770 cycles of microwave light absorbed or emitted by the hyperfine transition of cesium-133 atoms in their ground state undisturbed by external fields". So you don't have to worry about what the frequency you're measuring is, you just set up an apparatus to synchronize very closely with whatever it is, divide by 9,192,631,770, and that is by definition one second.
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 Sealed glass ampule, 99.98%.
A quite remarkable object, it contains about a gram of 99.98% pure cesium metal.
It's really very sad that if the glass were ever to break, it could very well explode on contact with the surrounding air blinding anyone in the area with flying shards of glass and molten metal. That's why we keep it under lock and key.
Here's a picture of the locking cover, which is immediately underneath the engraved tile for cesium:
Notice there are a couple of samples of gold in with the cesium. That's because we don't have a separate locking compartment for gold right now, and also because it's interesting to see how very similar the colors of gold and cesium really are.
Source: David Franco
Contributor: David Franco
Acquired: 11 June, 2002
Price: Donated
Size: 2"
Purity: 99.98%
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Sample from the Everest Element Set.
Up until the early 1990's a company in Russia sold a periodic table collection with element samples. At some point their American distributor sold off the remaining stock to a man who is now selling them on eBay. The samples (excepted gasses) weight about 0.25 grams each, and the whole set comes in a very nice wooden box with a printed periodic table in the lid.
To learn more about the set you can visit my page about element collecting for a general description and information about how to buy one, or you can see photographs of all the samples from the set displayed on my website in a periodic table layout or with bigger pictures in numerical order.
Source: Rob Accurso
Contributor: Rob Accurso
Acquired: 7 February, 2003
Price: Donated
Size: 0.2"
Purity: >99%
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